ISIS
The ISIS, if operational and not in need of reset, can on its own determine all the correct indications to display on its face (pix by A33Zab at post 1940), except that the indication of IAS and FL depend on the probe PITOT3 and the probes STATIC3 (same ref post 1935). [If these probes are giving erroneous readings, there is in theory some ability to detect gross divergence from speed made good and true altitude-- I cannot say if this is implemented in some way, or if this display can show that.] I do not know the effect of the loss of bus power; is there an internal floating battery or capacitor source, and of what duration?
It is reasonably clear that the inputs from ADIRU-1 and -3 are used to initialize ("reset" button) the ISIS before each flight (to the heading, pitch, roll, and altitude of the standing A/C), and that if these inputs are lost, an ISIS FAIL message to ACARS may be generated for maint, if for no other reason than possible loss of ability to make this AF SOP required reset at the next terminal. Obviously, in an emergency, a reset in flight into an erroneous input from either ADIRU needs to be avoided, and in general should be unnecessary.
So the questions are:
1. Is an inflight erroneous reset blocked by disagreement of ADIRUs 1 and 3? Or otherwise?
2. Does crew training include instruction on how, when, and when not to make inflight reset?
3. Does crew training include how and when to bring the ISIS up for flying, if needed?
3a. Or how, when, and why it becomes the instrument for flying, if automatic?
4. Does crew training include mention that all the ISIS displays are normally accurate (IMO), except that IAS and FL may err due to probe icing?
5. Is there any SIM training with ISIS as the only flight instrument? with its ILS feature?
6. [I omit mentioning again all the proper flying procedures covered by others.]
7. Does the QRH cover all this? Can or should it be memory trained?
8. Are electric power issues covered? LCD screen readable with flashlight if necessary (that external 5vac)?
And a final question:
9. If an ISIS FAIL message is generated for ACARS due ADIRU input fail (see above), does it blandly repeat on the PFDs in just those words? Can we tell from the ACARS? (I know... not easily.)
[I recall a respected pilot here saying we should have a TV record of just what appears on the PFDs. The point is, is there an ISIS FAIL on the PDFs, when in fact the ISIS is functional? And does the SIM replicate the A/C in this detail?]
BTW, the 3 gyros are not one operational gyro and 2 backups-- all 3 are operational with specific functions. That makes it self-sufficient as to A/C attitude. It isn't actually necessary to know more than this to know that it doesn't need the ADIRU inputs to generate its display.
Hope this helps. OE